The solubility in water or organic solvents of salts of cationic dyes and fluorescent brighteners depends not only on the dye or brightener cation, but especially also on the anion. For instance, the dye or brightener salts generally obtained on synthesis are in many cases not readily water-soluble; they are salts of inorganic acids, for example chlorides, sulfates or methosulfates. However, as salts of organic acids (for example as acetates) the same dyes or brighteners are significantly more water-soluble.
To prepare concentrated dye or brightener solutions, which are becoming commercially increasingly important as liquid brands, it is necessary to have dye or brightener salts which are readily water-soluble, since it is only in this way that the solvent content in the preparations can be kept as low as possible. But a high solubility of the dye or brightener is also of advantage in preparing solid preparations, since it makes possible a gentle and energy-saving drying, because highly concentrated starting solutions can be used.
In the past there has been no shortage of attempts to replace the anions of sparingly soluble cationic dye or brightener salts by the acid radical of carboxylic acids and thus to increase the water-solubility of the salts. For example, German Offenlegungsschrift No. 2,549,436 gives a process in which the inorganic acid radical of cationic dye salts is chemically bonded by means of epoxides. This chemical conversion is carried out in the presence of lower aliphatic carboxylic acids, the acid radical of which acts as a counterion and takes the place of the bonded anion. This process converts a dye chloride, for example, by means of epichlorohydrin in the presence of acetic acid into the corresponding dye acetate, and the epichlorohydrin is turned into propylenechlorohydrin.
This process has the disadvantage that the replaced anion becomes bonded in the form of an organic compound which cannot be put to any further use, and thus represents a loss of valuable organic material. The replacement of the anion by chemical means is also an additional reaction step, and hence, at the least, gives rise to the formation of undesirable by-products.
It is thus an object of the invention to find a process free of these disadvantages, i.e. to develop a process which can be carried out without problems and which has no adverse effect on the quality of the dye nor gives rise to useless organic products.
This object is surprisingly achieved with the aid of a membrane-separating process by means of Donnan dialysis (on the theory of the Donnan dialysis see S. T. Hwang and K. Kammermeyer, Membranes in Separations; Wiley & Sons, N.Y. 1975).
The present invention thus relates to a process for converting inorganic salts, such as the chlorides, sulfates or methosulfates, of cationic dyes and brighteners into more soluble salts of organic acids without intermediate isolation of the free base, which comprises replacing the inorganic acid radical of the dye or brightener by the anion of a cyclic or acyclic organic acid by means of Donnan dialysis.